In the processing of semiconductors, such as GaAs in this example, there are difficulties in measuring the temperature of wafers or of thin layers, especially in applications where the temperature has to be known accurately, and no physical contacts to the wafer (or thin layer) are permitted. Two examples of processes where these problems arise are "heat-cleaning" of wafers prior to growing subsequent layers on them by Molecular Beam Epitaxy, and preactivation "heat cleaning" of photocathodes.
A device which as been used in the past, in an attempt to overcome these difficulties is the pyrometer, which utilizes the black-body (or "gray body") radiation of the sample in order to measure its temperature. This method, however, is valid only when the wavelength of the radiation used is such that its characteristic coefficient of absorption is very large in comparison with the reciprocal of the thickness of the wafer or the thin layer. Such is rarely the case with wafers or thin layers of semiconductors such as GaAs, since the long wavelength light (.lambda.&gt; 1000 nanometers) used in pyrometry is hardly absorbed (if at all) by the semiconductor whose bandgap energy exceeds that of the light. Only for thick wafers, having temperatures well above room temperature, can the pyrometric method be applied: in these situations, wavelengths of about 900 nanometers are employed.
Pyrometers, therefore, when used in applications to GaAs or to semiconductors of comparable bandgaps, almost always monitor the temperature of the body on which the semiconductor wafer rests rather than the actual temperature of the semiconductor material. In the case of the photocathode bonded to a glass faceplate, the pyrometer (utilizing radiation well above 900 nanometers) absorbs radiation emitted by the glass faceplate. The cathode itself, which is totally transparent to such radiation, is not "seen" at all by the pyrometer; and, furthermore, the cathode layer introduces an additional complication by acting as a thin film interference filter. This latter effect causes the pyrometric temperature readings of the glass faceplate itself to be in error --depending on the thickness of the cathode layers. The thinner the cathode layer, the more sensitive the pyrometer reading to small variations in the layer's thickness.
It is sometimes possible to apply a transmission method to overcome the above limitations of the pyrometer. Indeed, the use of the transmission method has been described in some detail in U.S. Pat. No. 4,890,933 entitled TRANSMISSION METHOD TO DETERMINE AND CONTROL THE TEMPERATURE OF WAFERS OR THIN LAYERS WITH SPECIAL APPLICATION TO SEMICONDUCTORS issued on Jan. 2, 1990 to A. Amith et al and assigned to the assignee herein. In that patent, there is described a method of accurately determining the temperature of a thin layer of bandgap material without requiring contact to the layer. The method uses optical radiation through the layer and the detection of optical absorption by the layer. The relationship between the temperature varying bandgap energy and the resulting optical absorption characteristics provides an indication of temperature, independent of ambient temperature. Reference is also made to a copending patent application, Ser. No. 399,729 filed Aug. 28, 1989 as a divisional of the application resulting in the above-noted patent and having the same title, inventors and assignee. The present invention is based on the monotonic change in the optical absorption coefficient as a function of temperature. It furthermore depends on the condition that the light has an energy comparable to and/or slightly larger than the bandgap. In this regime, the reflectance will be a function of the absorption coefficient .alpha. as will be explained below. In the specific example to which the invention is applied herein, the controlling phenomenon is the narrowing of the bandgap of the semiconductor (it is the direct optical bandgap in the case of GaAs) with increasing temperature. Since the absorption coefficient .alpha. for light of a narrow spectral range whose photon energy is slightly higher than the bandgap energy, depends on the separation between these two energies, it follows that the absorption coefficient will depend on the temperature of the GaAs wafer or thin layer. The relationship between photon energy and the bandgap energy is shown in equation form in the specification. The energy of the narrow spectral range employed in this mode must be such that it stays near, yet above the band edge at the temperatures of interest (if, at any temperature, the bandgap exceeds the spectral energy, then the light will not be absorbed and therefore the absorption coefficient .alpha. will not be temperature-sensitive. Consequently, the optical reflectance will not be as temperature-sensitive as when it has the exponential dependence on .alpha.).
Indeed, this invention is applicable to all materials whose optical absorption coefficient is a monotonic function of temperature. It is applicable, in particular, to all semiconductors and is enhanced by selecting narrow optical spectral ranges very close to the respective bandgaps. The underlying mechanism is the same as detailed in this description of the invention as applied to GaAs: the absorption coefficient of optical radiation close to the bandgap (and exceeding the latter's energy by a small amount) is a function of the bandgap. Since in all semiconductors the bandgap is a function of temperature, the invention applies to all semiconductors. It further applies to semiconductors wherein the bandgap is either direct or indirect.
It is the object of the present invention to determine the exact temperature of the semiconductor thin layer or wafer, without contacting the semiconductor physically, and without having light transmitted through the semiconductor.
The invention is based on measurement of optical reflectance, utilizing a properly selected narrow band spectral range which has an energy near the bandgap and slightly above it. Under these circumstances, the reflectance will depend exponentially on the absorption coefficient .alpha.. The coefficient .alpha., in turn, will be a function of the bandgap. The bandgap, in turn, is a function of the temperature of the relevant semiconductor layer. Consequently, the optical reflectance will depend on the temperature of the layer or of the wafer.
The present invention does not only provide a method of accurate determination of the temperature, but it furthermore is employed -- through an electrical feedback loop -- to control said temperature by adjusting the power to the heating agent.
The method of this invention is applied by selecting a very narrow spectral range and measuring its reflectance off the wafer.
The invention contains a continuous comparison of the intensities of the light component reflected from the wafer, with that emitted by the light source. This "normalization" procedure enables one to separate those changes in reflectance off the wafer which are due to the latter's varying temperature, from changes which are due to variations in the intensity of the light source.
This invention overcomes many of the above-noted shortcomings and difficulties of existing pyrometric as well as transmission methods.
These and other objects of the invention are achieved according to the invention by providing a source of optical radiation having a desired spectral width and directing that optical radiation to a layer of material having a bandgap which varies as a function of temperature. The optical radiation reflected off the layer of semiconductor material is detected and analyzed to determine its value. Due to the relationship between direct bandgap and optical absorption coefficient .alpha., analysis of the reflected optical radiation will provide an indication of the direct bandgap of the material which, in turn, is indicative of the material's temperature.
For a semiconductor wafer or layer, an in-situ temperature determination may be accomplished while the wafer is in a heating chamber even though the temperature detection apparatus is maintained outside the heating chamber. Of course, the temperature detection apparatus could just as well be wholly or partially within the chamber if it is tolerant of the processing temperatures. A light source of a given spectral content may be provided. Since the absorption coefficient for this spectral range depends on the separation between the photon energy and the bandgap energy, it is possible to derive information relating to the bandgap by examining the reflectance off the wafer in the spectral range of interest. Additionally, the direct bandgap of GaAs narrows as temperature increases. Thus, information regarding the temperature of the GaAs wafer may be derived from the reflectance of the identified spectral range.